The overall objective of this project is to learn how the gonadotropins interact with their receptors to induce signal transduction. Previously we focused on identifying regions of the hormones that interact with their receptors. We identified portions of hCG and hFSH that control their abilities to distinguish LH and FSH receptors. Using an immunological strategy, we identified hormone residues that are exposed in the hormone receptor complex and could be detected by anti-hormone monoclonal antibodies. Combined with the crystal structure of deglycosylated hCG, these observations provide a model of how the hormone interacts with its receptor. Studies outlined in Specific Aims 1 and 2 will enable us to identify other portions of the hormone that are responsible for the receptor binding specificity of glycoprotein hormones. However, receptor binding is only the first event in signal transduction. While considerable information is available about the portions of the hormone that confer binding specificity, especially now that the crystal structure of deglycosylated hCG is known, much less is known about the events that occur to cause signal transduction. Aims 2 and 3 are designed to obtain information on the interactions of hCG with LH receptors that lead to signal transduction. Using a receptor analog that can be labeled, biotinylated analogs of hCG, and antibodies to the LH receptor, we will map the surfaces of hCG, deglycosylated hCG (an antagonist of hCG induced signal transduction), and the receptor. The studies employing biotinylated hCG should provide new information on hormone residues that are exposed after hCG binds to LH receptors. When applied to analogs of hCG that have hFSH activity, this technique should enable us to identify the different parts of the analog that interact with LH and FSH receptors. We are particularly interested in identifying specific portions of the hormone and the receptor that change following hormone binding and are associated with signal transduction. Therefore we will identify differences the portions of hCG and the receptor that are exposed after binding of hCG and deglycosylated hCG. Finally, we will obtain an independent estimate of the number of hormone and receptor molecules that are within the hormone receptor complex.